Process for generating periodic non-uniform electric field, and for removing polarizable particulate material from fluid, using ferroelectric apparatus

ABSTRACT

Polarizable particulate material, such as organic and inorganic colloidal particles such as small pieces of metal, oxides and the like, zwitterionic molecules, and even living organisms and viruses can be preferentially removed from a liquid by dielectrophoresis, by passing the liquid containing the polarizable particulate material to be removed over a ferroelectric apparatus which generates a periodic non-uniform electric field near the boundary between alternately polarized portions of the ferroelectric material. The periodic non-uniform electric field is generated by subjecting portions of the ferroelectric material to an alternating potential to alternately polarize the portions, while allowing other portions of the ferroelectric material to remain polarized in the same direction.

This is a division, of copending application Ser. No. 348,835, filedApr. 6, 1973.

BACKGROUND OF THE INVENTION

It is well known that if an electric field is generated between twooppositely charged electrodes, any charged particles which are locatedbetween the two electrodes will be attracted to the correspondingelectrode having a charge opposite that of the particle. This phenomenonis known as electrophoresis. If the particles do not have a net charge,and are polarizable, the electric field will induce a polarization inthe particle; for example in a metal particle this results in a shift ofelectrons to the side of the particle facing the positive electrode.Since the particle is still neutral, although polarized, no movementoccurs in a uniform field. The magnitude of the resultant polarizationis related to the effective dielectric constant of the material and ofthe support medium. Materials which have high dielectric constantsexhibit large polarizations in the presence of an electric field.Materials which have a low dielectric constant, on the other hand,develop much lower polarization in the presence of an electric field.

If the particles are placed between two oppositely charged electrodeswhich produce a non-uniform field, such as that produced when oneelectrode is a point or line and the other electrode is a plane, thepolarized particles will experience a net force tending to move theparticle into the region of higher electric field strength, assuming thedielectric constant of the particle is greater than that of the mediumin which it is located. It may be noted that the direction of the forceis not dependent upon the sign of the electric field but is always inthe direction of the field gradient. This motion of matter caused bypolarization effects in a non-uniform electric field is known as"dielectrophoresis". The principle of dielectrophoresis has beenutilized in various types of apparatus to remove polarizable particlesfrom fluids, by passing the fluid containing the polarizable particlesto be removed between two dissimilar electrodes, between which isgenerated a non-uniform electric field. The most popular electrodeconfiguration for such particle separation apparatus appears to be theconcentric cylinder configuration or some variant of it, such as asingle wire in the center of a surrounding cylindrical electrode. Otherelectrode configurations have been used, however. All of these variousprior art apparatuses appear to have in common the feature of passingthe fluid containing the polarizable particles to be separated betweentwo electrodes which have a configuration other than parallel planes, sothat a non-uniform electric field is formed between the electrodes whena current is applied to them.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process andapparatus for removing polarizable particulate material from a fluid.The apparatus comprises a ferroelectric material, polarizable indirections perpendicular to the surfaces of the ferroelectric material,as more fully hereinafter described; a plurality of electrodes, appliedto opposite sides of the ferroelectric material; a source of alternatingpotential, connectable to the electrodes; and means for positioning thefluid containing the polarizable particulate material to be removed inthe periodic non-uniform electric field which is generated by theapparatus, while the field is being generated. The ferroelectricmaterial, electrodes and source of alternating current are arranged suchthat the ferroelectric material comprises at least one portion,preferably a plurality of portions, the direction of polarization ofwhich is to be alternated during generation of the periodic non-uniformfield. The ferroelectric material also comprises at least one portion,and again preferably a plurality of portions, the direction ofpolarization of which is to remain the same during the generation of theperiodic non-uniform electric field.

More generally, the present invention also provides an apparatus andprocess for generating a periodic non-uniform electric field external tothe field-generating electrodes, whether for use in a particleextraction apparatus or for other purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of one type of dielectrophoresisparticle extraction apparatus, in accordance with the present invention.

FIGS. 2, 4, 6, 8, and 10 are enlargements of a portion of the electrodedferroelectric material of FIG. 1, illustrating the directions ofpolarization and field strength under the various conditions of appliedfield.

FIGS. 3, 5, 7, 9 and 11 are illustrations of the hysteresis loop whichis characteristic of the ferroelectric material of FIG. 1, showing thecondition of applied field, E, and polarization, P, for FIGS. 2, 4, 6, 8and 10, respectively.

In particular, FIGS. 2-3 illustrate the condition in which no field ispresent inside or outside the element.

FIGS. 4-5 illustrate the condition in which the positive field, lessthan the coercive field (E_(c)) is applied to the element, so that nopolarization switching has yet occurred.

FIGS. 6-7 illustrate the condition in which a positive field, greaterthan the coercive field E_(c), is applied to the element, so that thedirection of polarization under the electrodes is completely switched.

FIGS. 8-9 illustrate the condition in which an even greater positivefield is applied to the element, so that significant switching isinduced beyond the edge of the electrodes.

FIGS. 10-11 illustrate the condition in which the applied field is againreduced to 0.

FIG. 12 is a schematic isometric illustration of an electrodeconfiguration for another form of apparatus according to the presentinvention, for removing polarizable particulate material from a fluid.

FIG. 13 is a second view of the configuration of FIG. 12, taken alongline 13--13 of FIG. 12, and illustrating additional components.

FIG. 14 is a schematic plan view of another form of apparatus accordingto the present invention, for generating a periodic non-uniform externalelectric field.

FIG. 15 is a schematic view of another configuration in accordance withthe present invention, wherein portions of the ferroelectric material,the direction of polarization of which is to remain the same duringgeneration of the periodic non-uniform electric field, are polarized inalternate directionns perpendicular to the surface of the ferroelectricmaterial.

FIG. 16 illustrates another configuration in accordance with the presentinvention, wherein alternate electrodes are slightly overlapped onopposite sides of the piece of ferroelectrode material.

FIGS. 17 and 18 illustrate a configuration of assembling a plurality ofpieces of ferroelectrode material in an apparatus for removingparticulate polarizable materials from a fluid.

FIG. 19 illustrates an alternative method of forming electroded andnon-electroded portions of the surface of the ferroelectric material.

FIGS. 20, 21, and 22 illustrate additional configurations of electrodesand ferroelectric material which can be utilized in the presentinvention. FIGS. 23-25 together illustrate yet another configuration ofelectrodes and ferroelectric material for use in an apparatus inaccordance with the present invention.

Throughout the drawings the following conventions are used: Arrows withconical heads indicate fluid flow. Solid arrows with ordinary headsindicate directions of polarization. Double-headed arrows indicatealternated directions of polarization. Dotted arrows indicate electricfields.

DETAILED DESCRIPTION

This invention relates to an apparatus and process for generating aperiodic non-uniform electric field, and to an apparatus and process forremoving polarizable particulate material from a fluid, using theapparatus for generating a periodic non-uniform electric field as anelement of the apparatus for removing polarizable particulate materialfrom a fluid.

By a "non-uniform" electric field, it is meant that the lines of forceof the electric field are not parallel, and are therefore moreconcentrated, and the electric field is stronger, in one location thanin another. By "periodic" it is meant that the non-uniform electricfield is not of constant strength, but becomes stronger and weaker andreverses at various times, passing through a repeated sequence of suchvalues. By "external" it is meant that the place where the periodicnon-uniform electric field is generated is located other than betweenthe electrodes which are used to generate the periodic non-uniformelectric field.

The central element of the apparatus for generating a periodicnon-uniform electric field is a ferroelectric material which ispolarizable in directions perpendicular to the surface of theferroelectric material. "Ferroelectric materials" (or simply"ferroelectrics") are a sub-set of the class of pyroelectric materials,and they have been used for a variety of sophisticated electronicequipment. A ferroelectric material is a pyroelectric material whosepolarization can, as a consequence of the crystallographic structure ofthe ferroelectric, be reversed or reoriented by application of asuitably directed electric field of sufficient magnitude. The electricfield needed to switch the polarization is a characteristic of theparticular ferroelectric utilized. The electric field necessary toswitch the direction of polarization is known as the "coercive field"(E_(c)) of the ferroelectric material and may vary with the direction ofthe crystal orientation with respect to the field direction when thecoercive field is measured. In most ferroelectric crystals and ceramicsthe coercive field is also a function of both form and frequency of theapplied field. In fact, depending on the crystallographic structure ofthe particular ferroelectric material involved, there may not be acoercive field defined in a particular direction because it may be thatno amount of electric field applied in that direction will cause achange in the direction of polarization of the ferroelectric material.

Ferroelectric materials which can be utilized for the present inventioninclude various known ferroelectric materials such as barium titanate,triglycine sulfate ("TGS"), triglycine fluoberylate ("TGFB"), bariumniobate, strontium niobate, and sodium potassium tartrate tetrahydrate(Rochele salt).

Solid solutions made from suitable ferroelectric end members such asthose listed below, can also be employed.

     TGS - TGFB                                                                   BaTiO.sub.3 - SrTiO.sub.3                                                     PBTiO.sub.3 - PbZrO.sub.3                                                     BaNb.sub.2 O.sub.6 - SrNb.sub.2 O.sub.6                                       BaNb.sub.2 O.sub.6 - SrTa.sub.2 O.sub.6                                   

For suitable compositions such as PbTiO₃ -PbZrO₃ the ferroelectric mayalso be used in polycrystalline ceramic form.

For use in the present invention, ferroelectric material polarizable indirections perpendicular to the surface of the ferroelectric material isneeded. Ferroelectric materials can have one or more "ferroelectricaxes", or directions in which polarization of the ferroelectric materialcan exist. Some crystals have only a single ferroelectric axis, in whichcase the crystal can exhibit spontaneous polarizations in only twodirections (each direction along the single axis); other crystals havemultiple ferroelectric axes, so that spontaneous polarization can existin several directions. Either type of ferroelectric material, eitherhaving a single axis or multiple ferroelectric axes, is useful for thepresent invention.

Before describing in detail the circuit of the apparatus for generatinga periodic non-uniform electric field in accordance with the presentinvention, one type of apparatus for removing polarizable particulatematerial from a fluid which utilizes this circuit will be brieflydescribed with reference to FIG. 1.

Referring now to FIG. 1, it is a sectional schematic view of adielectrophoresis particle extraction apparatus in accordance with thepresent invention, i.e., an apparatus for removing polarizableparticulate material from a fluid. This apparatus comprises a piece ofplanar ferroelectric material 21 bearing electrodes 22 and 23, planarferroelectric material 21 serving as one wall of a conduit through whichfluid containing polarizable particulate material to be removed is past.The remaining walls 24 of the conduit complete the basic apparatus,except for the electric circuitry described below. In order to utilizethis apparatus, the fluid containing the polarizable particulatematerial to be removed is positioned in the periodic non-uniformelectric field while the periodic non-uniform electric field is beinggenerated, in this case by passing the fluid through the periodicnon-uniform electric field which is located in the positions betweenelectrodes 22. The path of the fluid through the conduit defined byplanar ferroelectric material 21 and walls 24 is indicated by arrows 25.

To illustrate the influence of the ferroelectric material in generatingthe very high field gradients necessary for effective dielectrophoreticseparation, reference is made to FIGS. 2-11. In these figures, thedirection of polarization of the portions of the ferroelectric materialare indicated by unbroken arrows, and the direction of field strength isindicated by dotted arrows. It is assumed that before electroding, theferroelectric crystal (or ceramic) was poled to negative remanence, asshown by the working point a in FIG. 3, then left to equilabrate. Aftera time which is long compared to the dielectric relaxation time tau ofthe ferroelectric material 21, the charge situation at the surfaces offerroelectric material 21 will be as shown in FIG. 2. Unbroken arrows 27illustrate the spontaneous polarization of ferroelectric material 21with bound charges 28 and 28' on the surfaces of the ferroelectricmaterial 21. These bound charges will be exactly compensated by "free"surface charges 29 and 29', and no field will exist either inside oroutside the piece of ferroelectric material 21.

If electrodes 22 and 23 are now connected to a generator which begins togenerate a positive field between electrodes 22 and 23, as illustratedin FIG. 4, some positive charge will be applied to electrode 23,negative charge to electrode 22, and a field will be set up in thedirection opposing the spontaneous polarization P_(s) (unbrokenpolarization arrows 27), as shown by the dotted arrows 30 in FIG. 4. Ifthe voltage applied to electrodes 22 and 23, shown as single cell DCpotential source 31, is less than the coercive field E_(c) of theferroelectric material 21, the working point will now move out on thehysteresis loop (FIG. 5) to some point b. Electric field as shown bydotted arrows 30 and 30' will now exist both inside and in the spaceoutside the ferroelectric material 21, and a strong field gradient(dotted arrows 30") begins to develop at the edge 32 of electrode 22, onthe upper surface of ferroelectric material 21.

Of special importance, however, is the field at the edge 32 but insidethe ferroelectric material 21. It can be seen from FIG. 4 that becauseof the equipotential of the electrode on the lower surface offerroelectric material 21, the field will tend to spill out (fringe)into the unelectroded portion of the ferroelectric material 21 (seedotted field arrow 30'" of FIG. 4).

Referring now to FIGS. 6 and 7, if the field is further raised beyondthe coercive field E_(c) necessary for ferroelectric switching, asillustrated by triple cell DC potential source 33 in FIG. 6, for exampleto point c in FIG. 7, the spontaneous polarization P_(s) under electrode22 will now invert (see polarization arrows 27 of FIG. 6). It should benoted that while FIGS. 4, 6 and 8 illustrate the potential applied toelectrodes 22 and 23 as DC potential sources, this is merely anindication of the instantaneous potential condition which is in realitysupplied by an alternating source of potential.

Massive negative charge must now flow onto electrode 22 to compensatethe switching charge associated with the spontaneous polarization whichis indicated by polarization arrows 27. However, spillover of the field,together with the continuity of the domain wall, will force someswitching beyond edge 32 (see polarization arrow 27' of FIG. 6). Since,however, there is no electrode on this surface above polarization arrow27', and therefore no contact to the source of potential 33, no freecharge will accumulate to compensate the large bound positive charge ofthe domain switching.

This large positive charge now forms a virtual electrode (accumulationof charge which behaves as an electrode) extremely close to the truenegative electrode at edge 32, driving up the surface field to very highvalues and producing an exceedingly large field gradient at edge 32, asshown by field arrows 30" in FIG. 6.

One cardinal advantage of this virtual electrode is shown in FIGS. 8 and9. If the E field at the surface of the ferroelectric material 21 is nowincreased further, for example to working point d in FIG. 9, so thatfurther fringe switching is induced (see polarization arrow 27" in FIG.8), then the E field at the surface at the point indicated by number 34,near the edge 32 of the real electrode, may exceed the breakdownstrength of the ambient medium. In this case, carriers will betransported from the negative electrode over the surface. In a normalelectroded device, catastrophic short circuit would result. For thecircuit under consideration, however, the accumulating negative surfacecharge simply moves the high field region over, new switching isinduced, and a new high field region is generated at point 34.

Referring now to FIGS. 10 and 11, when the source of potential hascycled to the point where no field is applied to electrodes 22 and 23,as illustrated by working point e of FIG. 11, the polarization andcharge situation will be as shown in FIG. 10. Both the internal andexternal fields will be absent at this point, and the only chargespresent will be the bound polarization charges 28 and 28' and thecompensating free surface charges 29 and 29'. A new half cycle may nowbe initiated, with the working point moving around the hysteresis loopat points on the left hand (negative field) side, corresponding to b, cand d in FIGS. 5, 7, and 9, on the right hand side of the hysteresisloops in these figures. The field and polarization effects for this halfof the cycle are exactly opposite to those of the half of the cycleillustrated in FIGS. 4-11, with the high intensity field having the sameshape but merely the opposite direction.

It may be noted that:

1. The highest field is generated immediately adjacent to the electrodeedge 32.

2. The field strength drops away rapidly into the ambient medium, withdistances further from electrode edge 32.

3. The sign of the field inverts on each half cycle of the drivingfield.

4. The field gradient, unlike the field itself, does not change sign, sothat the gradient E² which is responsible for the dielectrophoreticdriving force for particle removal goes from zero to its maximum valuetwice on each cycle of the driving electric field.

5. Breakdown or limited conduction in the ambient medium only serves totransport the region of maximum field (and of the highest fieldgradient) along the surface of the ferroelectric material 21.

The invention will now be illustrated with two examples.

EXAMPLE 1

To demonstrate the high field, field gradient and particle separation,referring now to FIG. 12, a piece of ferroelectric material 36 (bariumtitanate) was mounted upon an alumina substrate 37. The piece of planarferroelectric material 36 was approximately 50 mm by 1.6 mm and 0.17 mmthick. To the bottom of the piece of planar ferroelectric material 36 asillustrated in FIG. 12, a metallized silver electrode was applied over a25 mm length center section (not shown in FIG. 12). To the top surfaceof the piece of planar ferroelectric material 36 was applied a pair ofsilver electrodes 38, having a thin gap of about 0.25-1.0 mm width. Thepiece of planar ferroelectric material 36 was then cemented with silverepoxy cement to the alumina substrate 37, suitable electrodes (not shownin FIG. 12) were applied, and the assembly was then enclosed between twoglass plates 40 (see FIG. 13). In one end of the cell was mounted amicroscope illuminating lamp, and at the other end of the cell atelemicroscope with camera attachment was mounted. Electrical fieldswere obtained from an audio power amplifier driven by a General Radiounit oscillator, and the amplitude of the voltage applied to electrodes38, on the one hand, and the electrode on the bottom of the piece ofplanar ferroelectric material 36 on the other hand, was monitored with aTektronix oscilloscope and a Hewlett-Packard vacuum tube voltmeter.Glass plates 40 were secured with a room temperature vulcanizingsilicone cement.

The completed cell was then filled with a liquid containing acicularpolarizable particles suspended in a nonionic insulating liquid. Thisliquid was Marks Polarized Corporation's Varad Electroopticall Fluid No.V102. This liquid contains needle-like crystalline polarizable particlesin a phthalic acid ester base. When randomly distributed, the fluid isopaque, but at well defined field strength and frequency, the crystalscan be reoriented parallel to the field and the fluid becomes clear.

A frequency of 4500 cycles alternating potential of a variable voltagewas applied to the electrodes on both sides of the planar ferroelectricmaterial 36. As voltage was gradually increased, a small clear areadeveloped about the gap in the top surface electrode 38. This area grewlarger with increased voltage and became smaller as the voltage wasdecreased, and the effect was found to be reproducible. In a second testin similar apparatus, as the voltage was increased, a halo was observedin the vicinity of the gap 39 between the two electrodes 38. Because theplanar ferroelectric material had not been fastened securely to thealumina substrate 37 along its entire length, light could be transmittedbetween the planar ferroelectric material 36 and the alumina substrate37. At zero volts this area was quite dark. Increasing the voltage to225 volts (root mean squared) resulted in a clearing of the area. Theclear area grew larger at 350 vrms applied to the test cell.

The effect of frequency was not quite as pronounced. Applying a voltageof 225 vrms on the cell, a larger clear area was observed with afrequency of 45000 cycles than was observed with 450 cycles. Each ofthese observations was reproducible, as well as the halo effect in thegap area.

As a control, a test cell of similar design, but with a glass stripsubstituted for the planar ferroelectric material 36, was subjected tothe same voltage conditions. With 50 vrms applied to the cell, nooptical effect was noted. At 100 vrms heating and bubbling of theelectrooptical fluid were the only effects observed.

The test was repeated again with a second glass test cell. Again at 30vrms no optical effect was noted. This was also true at 45000 cycles andat 450 cycles. With 100 vrms applied, slight movement of the particleswas observed, indicating a dielectric heating effect. Increasing thevoltage to 150 vrms resulted in rapid boiling of the fluid.

It was noted for the ferroelectric cells that after a time of the orderof 5 minutes under alternating potential of 225 vrms, the efficiency ofthe cell as judged by the halo about the electrode gap began todiminish. Stopping the experiment, a thin deposit was observed in thegap region. On removing this deposit with a soft camel's hair brush, thefull efficiency was restored. This process was repeated several times,indicating that the strong field gradient was causing the acicularsemiconducting particles to be removed from the Marks fluid.

EXAMPLE 2

Illustrating the extension of ferroelectric switching into the gapregion between electrodes, a piece of planar ferroelectric material,polarizable in directions perpendicular to the plane of the planarferroelectric material, specifically a lead zirconate - lead titanateproduced by Vernatron Piezoelectric, Inc., and identified by them asPZT-5H, was prepared. This piece was 12.7 mm by 15.1 mm and 0.17 mmthick. On this piece of ferroelectric material 41 (see FIG. 14) a seriesof three electrodes 42-44 were applied to the upper side piece offerroelectric material 41. Electrode 42 was rectangular, approximately2.4 mm by 7.1 mm. Electrode 43 was also rectangular, approximately 3.2mm by 7.1 mm. A space between electrodes 42 and 43 of about 0.25 mm wasprovided. Midway between electrodes 42 and 43, a narrow strip electrode,the upper portion of electrode 44 as illustrated in FIG. 14, 0.038 mm inthickness, was applied. This electrode was in its entirety of a T-shapeas illustrated in FIG. 14.

In operation, electrodes 42 and 43 were connected to an AC generator of800 volts rms at 60 Hz. Electrode 44 was connected to a 0.22 microfaradintegrating capacitor and then directly to the Y-amplifier of acathode-ray oscilloscope. For this oscilloscope, the x-deflection wasprovided by a tap from the 60 Hz supply to electrodes 42 and 43.

The appearance of a hysteresis loop on electrode 44 clearly indicatedthe occurrence of switching of the ferroelectric material 41 within thegap region between electrodes 42 and 43, driven by the fringe field fromelectrodes 42 and 43. From the quantity of charge switched (about 0.053microcoulombs, over an area of 0.0028 cm²), it is clear that some 20microcoulombs/cm², a major fraction of the spontaneous polarizationP_(s) in this PZT-5H material, was being switched.

Other configurations of ferroelectric material can be used, such assuccessive portions 21a, 21b, 21c, 21d of planar ferroelectric material21 (see FIG. 15) between successive electrodes 22 being polarized inalternate directions perpendicular to the plane of the planarferroelectric material. This configuration may have advantages insituations in which a number of parallel plates of ferroelectricmaterial 21 are arranged in close array, so that the overlappingelectric fields from adjacent plates can be correlated in theirdirections. In FIG. 15, as in FIG. 16, the portions of the ferroelectricmaterial whose direction of polarization is alternated are indicatedwith double-headed arrows, and the directions of polarization of thoseportions of the ferroelectric material which is not alternated areindicated by singleheaded arrows.

Another configuration is shown in FIG. 16, wherein alternate slightlyoverlapping electrodes 22, 23 are placed on opposite sides offerroelectric material 21. An advantage of the configuration shown inFIG. 16 is that the high intensity fields can be generated on both sidesof ferroelectric material 21, and in fact a plurality of plates offerroelectric material 21 as illustrated in FIG. 16 can be arranged injuxtaposition as shown in FIG. 17. Using this configuration, it ispossible to connect certain of the electrodes to separate sources ofalternating potential, 26', 26", 26'", so that the various differingportions of the array of ferroelectric materials 21 can be subjected todifferent phase conditions, and a proper cooperation of the highintensity field locations can be obtained; or so that by utilizingdifferent frequencies, different types of particles which are moreresponsive to different frequencies can be selectively collected indifferent locations of the apparatus.

The parallel plate configuration as shown in FIG. 17 can then be placedin a suitable conduit 47 (see FIG. 18), through which the fluid to bepurified can be passed as shown by arrows 48.

The electrodes need not be in the configuration of parallel stripes, anda polka-dot configuration of nonelectroded portions is illustrated inFIG. 19. The nonelectroded portions 50 can be formed by high spots inthe ferroelectric material, so that the entire surface of theferroelectric material is initially electroded (such as by flashing ongold metallic electrodes by conventional technology), following whichthe high portions are polished off to remove the unwanted portions ofthe electrode material. The irregular dimpled shape of the ferroelectricmaterial necessary to produce this configuration can either be formed bypressing the uncured ferroelectric material, prior to firing andpolarization, or by sandblasting the cured ferroelectric material afterfiring. In either case, the high portions of ferroelectric material 21which is covered by electrode 22 are polished off leaving exposedportions 50 of ferroelectric material 21, which can serve as local citesof particle collection.

Another variant of the apparatus illustrated in FIG. 18 is shown in FIG.20. Instead of parallel sheets of ferroelectric material, theferroelectric material with alternate striped electrodes can be arrangedin a spiral.

Another approach to using a tube as the conduit is illustrated in FIG.21, wherein the walls of the conduit are themselves the ferroelectricmaterial, with electrodes being applied inside and outside the conduit.Although the configuration of FIG. 21 is shown with electrodes 22running parallel to the axis of the ferroelectric conduit 21, otherconfigurations are also possible, such as parallel annular electrodes orspiral electrodes, inside or outside ferroelectric conduit 21.

Another approach which can be taken in the construction of ferroelectricbodies for use in the present invention is illustrated in FIG. 22. Inthis embodiment, small particles of ferroelectric material are sinteredlightly together, with random directions of polarization of theindividual particles. The porosity of the body as a whole is maintained,so that the liquid to be purified can be passed between the particles,for example from top to bottom as illustrated in FIG. 22. The electrodescan be applied to the surface of the ferroelectric material 21 in a wiremesh configuration, see electrodes 22 of FIG. 22. This allows thepassage of fluid directly through the electrodes. Additional electrodes(not shown in FIG. 22), likewise in wire mesh configuration, can beapplied to the bottom of the sintered ferroelectric material 21. Thisconfiguration has the advantage of combining conventional filtertechnology with the ferroelectric dielectrophoretic particle removingmethod according to the present inventon.

Yet another scheme is illustrated in FIGS. 23-25. A number of individualplates can be fabricated as per FIGS. 23 and 24, each plate consistingof a piece of ferroelectric material 21 which has deposited upon itelectrodes 22, 23. For use in this configuration, it is convenient toplace a palladium or platinum material on the uncured ferroelectricmaterial 21, together with a material over electrode 22 which can laterbe leached out by appropriate chemical action. The ferroelectricmaterial 21 and the electrodes 22, 23, together with the material to beleached (not shown in FIG. 24, but present between legs 21e, 21f offerroelectric material 21), can all be cured together at the same time.A plurality of these plates can then be assembled together in a sandwichconfiguration as illustrated in FIG. 25, and the material between legs21e and 21f can be leached out to form a passage for the fluid to bepassed through the apparatus as illustrated by arrows 48 in FIG. 25.

We claim:
 1. A process for generating a period non-uniform electricfield, external to field-generating electrodes of the apparatus forgenerating the periodic non-uniform electric field, in an apparatuscomprisinga. a ferroelectric material, polarizable in directionsperpendicular to the surface of the ferroelectric material; theferroelectric material comprising at least one portion, the direction ofpolarization of which is to be alternated during the generation of theperiodic non-uniform electric field, and at least another portion, thedirection of polarization of which is to remain the same during thegeneration of the periodic non-uniform electric field; b. a plurality ofelectrodes, applied to opposite sides of the ferroelectric material,covering both sides of the portions of the ferroelectric material, thedirection of polarization of which is to be alternated during thegeneration of the periodic non-uniform electric field, but leavinguncovered at least one side of the portions of the ferroelectricmaterial so as to produce a non-uniform electric field, the direction ofpolarization of which is to remain the same during generation of theperiodic non-uniform electric field, defining a location external to thefield-generating electrodes and adjacent the boundary between thealternately polarized portions of the ferroelectric material, in whichthe periodic non-uniform electric field is to be generated; and c. meansfor providing alternating potential to the electrodes1. for alternatingthe direction of polarization of the covered portions of theferroelectric material and leaving polarized in their originaldirection, the uncovered portions of the ferroelectric material;
 2. forproducing periodically alternately polarized portions of theferrolectric material; and
 3. for generating a periodic non-uniformelectric field external to the field-generating electrodes, in thelocation adjacent the boundary between the alternately polarizedportions of the ferroelectric material, said process comprisingproviding alternating potential to the electrodes on the ferroelectricmaterial in such a waya. as to alternate the direction of polarizationof the portions of the ferroelectric material, the direction ofpolarization of which is to be alternated, leaving polarized in theiroriginal direction the uncovered portions of the ferroelectric material,the direction of polarization of which is to remain the same; b. as toproduce thereby, periodically, alternately polarized portions of theferroelectric material; and c. as to generate thereby, a periodicnon-uniform electric field, external to the field-generating electrodes,in the location adjacent the boundary between the alternately polarizedportions of the ferroelectric material.
 2. A process for removingpolarizable particulate material from a fluid in a ferroelectricapparatus as described in claim 1, comprising the steps ofa. passing thefluid containing the polarizable particulate material to be removedthrough the periodic non-uniform electric field while the periodicnon-uniform electric field is being generated, whereby to concentratethe polarizable particulate material in locations of the most intenseperiodic non-uniform electric field; and b. periodically discontinuingthe generation of the periodic non-uniform electric field, and flushingthe apparatus with a cleaning fluid, whereby to remove the concentratedpolarizable particulate material from the apparatus.